Hematopoietic cell transplantation (HCT) provides a potentially curative treatment for a wide variety of diseases. HCT, however, is complicated by high incidence of transplant-related mortality, graft failure and graft versus host disease (GvHD). The interaction of stromal derived factor-1 (SDF-1) with CXCR4 chemokine receptor plays an indispensable role in hematopoietic stem cell homing and engraftment. We hypothesize that blocking the SDF-1/CXCR4 interaction with a specific CXCR4 antagonist would selectively enhance donor cell reconstitution in allogeneic HCT. Plerixafor is a highly specific and reversible antagonist of CXCR4 and will be used in the current study. Our recent studies in a congeneic mouse transplant model demonstrated that post-transplant administration of plerixafor significantly improved animal survival and selectively enhanced donor cell engraftment. This selective enhancement of donor cell reconstitution results from combined effects of mobilization of residual recipient stem cells by plerixafor and selective survival advantage of donor stem cells. The objectives of this proposal are to perform pivotal translational studies to move our study into a phase I/II clinical trial at the end of this award and to further dissect the mechanisms of plerixafor and the regulation of CXCR4 signaling. We have 2 specific aims. Our Aim 1 is to investigate the efficacy of plerixafor in enhancing donor cell engraftment in several allogeneic mouse transplant models that are directly relevant to clinical applications. Our Aim 2 is to further dissect the mechanisms through which plerixafor enhances donor cell reconstitution and to understand the regulation of CXCR4 signaling. Successful accomplishment of these aims will have important implications in HCT and will benefit patients with HCT. Furthermore, our study will shed new lights into the role of CXCR4 in hematopoietic stem cell homing, mobilization and expansion, as well as the regulation of CXCR4 signaling.